A - D

absolute humidity The weight of water vapour in a gas-water vapour mixture per unit volume of space occupied.

absolute measurement A measured value expressed in terms of fundamental standards of distance, mass and time.

absolute pressure The pressure measured relative to zero pressure (vacuum).

absolute stability A linear system is absolutely stable if there exists a limiting value of the Open loop gain such that the system is stable for all lower values of that gain and unstable for all higher values.

absolute value error The magnitude of the error disregarding the algebraic sign or, if a vector error, disregarding its direction.

absolute viscosity A measure of the internal shear properties of fluids expressed as the tangential force per unit area at either of two horizontal planes separated by one unit thickness of a given fluid, one of the planes being fixed and the other moving with unit velocity.

absorbance An optical property expressed as log (SIT), where T is the transmittance.

absorptance The fraction of the incident light absorbed.

absorption-emission pyrometer An instrument for determining gas temperature by measuring the radiation emitted by a calibrated reference source both before and after the radiation passes through the gas, where it is partly absorbed.

absorption hygrometer An instrument for determining water vapour content of the atmosphere by measuring the amount absorbed by a hygroscopic chemical.

absorption tower A vertical tube in which a gas rising through a falling stream of liquid droplets is partially absorbed by the liquid.

accelerometer A transducer used to measure linear or angular acceleration.

access time The interval between a request for stored information and the delivery of the information; often used as a reference to the speed of memory.

accuracy The ratio of the error to the full-scale output or the ratio of the error to the output, as specified, expressed in percent.

acidity Represents the amount of free carbon dioxide mineral acids and salts which hydrolise to give hydrogen ions in water. pH is the measure of hydrogen ions concentration.

ACK Transmission control character transmitted by a receiving device as an affirmative response to a sending device.

acoustical ohm The unit of measure for acoustic resistance, reactance or impedance; it equals unity when a sound pressure of one microbar produces a volume velocity of one cubic centimetre per second.

acoustic compliance The reciprocal of acoustic stiffness.

acoustic dispersion Separation of a complex sound wave into its various frequency components, usually due to variation of wave velocity in the medium with sound frequency; usually expressed in terms of the rate of change of velocity with frequency.

acoustic impedance The complex quotient obtained by dividing sound pressure on a surface by the flux through the surface.

acoustic inertance A property related to the kinetic energy of a sound medium which equals Za/2 7rf, where Za is the acoustic reactance and f is sound frequency; the usual units of measure are g/cm4. Also known as "acoustic mass."

acoustic radiometer An instrument that measures sound intensity by determining unidirectional steady state pressure when the sound wave is reflected or absorbed at a boundary.

acoustic sensitivity The output of a transducer (not due to rigid body motion) in response to a specified acoustical environment. This is sometimes expressed as the acceleration in g rms sufficient to produce the same output as induced by a specified sound pressure level spectrum having an overall value of 140 dB referred to 0.0002 dyne per sq cm rms.

acoustic stiffness A property related to the potential energy of a medium or its boundaries which equals, where Za is the acoustic reactance and is sound frequency; the usual units of measure are dyne/cm.

actuator A device responsible for actuating a mechanical device such as a control valve.

actuator, double acting An actuator in which the power supply acts both to extend and retract the actuator stem.

actuator, electric type A device which converts electrical energy into motion.

actuator, electrohydraulic type A self-contained device which responds to an electrical signal, positioning an electrically operated hydraulic pilot valve to allow pressurised hydraulic fluid to move an actuating piston, bellows, diaphragm or fluid motor.

actuator, electromechanical type A device which uses an electrically operated motor-driven gear train or screw to position the actuator stem. May operate in response to either analogue or digital electrical signals

actuator, fluid motor type A fluid powered device which uses a rotary motor to the actuator stem actuator, hydraulic type A fluid device which converts the energy of an incompressible fluid into motion

actuator, piston type A fluid powered device in which the fluid acts upon a movable cylindrical member, piston, to provide linear motion to the actuator stem

actuator, pneumatic A device which converts the energy of a compressible fluid, usually air, into motion.

actuator, single acting An actuator in which the power supply acts in only one direction. In a spring and diaphragm actuator, for example, the spring acts in a direction opposite to the diaphragm thrust.

actuator, vane type A fluid-powered device in which the fluid acts upon a movable pivoted member, the vane, to provide rotary motion to the actuator stem.

ADA A Pascal based, real time systems programming language developed for the United States Department of Defence.

adaptive control A control system which adjusts its response to its inputs based on its previous experience.

adaptive gain control A control technique which changes a feedback controller's gain based on measured process variables or controller set points.

adaptive tuning In a control system, a way to change control parameters according to current process conditions.

adiabatic Referring to a process which takes place without any exchange of heat between the process system and another system or its surroundings.

adsorption The concentration of molecules of one or more specific elements or compounds at a phase boundary, usually at a solid surface bounding a liquid or gaseous medium containing the specific element or compound.

agglomeration Any process for converting a mass of relatively fine solid material into a mass of larger lumps.

air bubbler liquid level detector A device for indirectly measuring the level of liquid in a vessel especially a corrosive liquid, viscous liquid or liquid containing suspended solids; it consists of a standpipe open at the bottom and closed at the top, which is connected to an air supply whose pressure is maintained slightly above maximum head of liquid in the vessel; air bubbles out of the bottom of the pipe, maintaining the internal pressure equal to the head of liquid in the vessel, pressure being measured by a simple gauge or transducer.

algorithm A prescribed set of well defined rules or processes for the solution of a problem in a finite number of steps

alias When varying signals are sampled at equally spaced intervals, two frequencies are considered to be aliases of one another if they cannot be distinguished from each other by an analysis of their equally spaced values.

aliasing False signals in the frequency domain caused by a measuring rate for digitising that is too slow.

alkalinity Represents the amount of carbonates, bicarbonates, hydroxides and silicates or phosphates in the water and is reported as grains per gallon, or ppm, as calcium carbonate.

ambient A surrounding or prevailing condition, especially one that is not affected by a body or process contained in it.

ambient air 1. Air to which the sensing element is normally exposed. 2. The air that surrounds the equipment. The standard ambient air for performance calculations is air at 80°F, 60% relative humidity, and a barometric pressure of 29.921 in. Hg, giving a specific humidity of 0.013 lb of water vapour per lb of air.

ambient conditions The conditions (pressure, temperature, etc.,) of the medium surrounding a given device or equipment.

analogue Pertaining to data in the form of continuously variable physical quantities. Contrast with digital. A waveform is analogue if it is continuous and varies over an arbitrary range.

analogue back up An alternate method of process control by conventional analogue instrumentation in the event of a failure in the computer system.

analogue control Implementation of automatic control loops with analogue (pneumatic or electronic) equipment.

analogue signal An analogue signal is a continuously variable representation of a physical quantity, property, or condition such as pressure, flow, temperature, etc.

analogue simulation The calculation of the time or frequency domain response of electrical circuits to input stimulus. It assembles and solves a set of simultaneous equations associated with circuit topology.

angle of repose A characteristic of bulk solids equal to the maximum angle with the horisontal at which an object on an inclined plane will retain its position without tending to slide; the tangent of the angle of repose equals the coefficient of static friction.

angle valve A valve design in which one port is colinear with the valve stem or actuator, and the other port is at right angles to the valve stem.

angstrom A unit of length defined as 1/643 8.4696 of the wavelength of the red line in the Cd spectrum; largely replaced by the SI unit nanometer, or 10 9 meters.

angular momentum The product of a body's moment of inertia and its angular velocity.

angular momentum flowmeter A device for determining mass flow rate in which an impeller turning at constant speed imparts angular momentum to a stream of fluid passing through the meter; a restrained turbine located just downstream of the impeller removes the angular momentum, and the reaction torque is taken as the meter output. Also called an "axial flowmeter."

angular velocity Rate of motion along a circular path, measured in terms of angle traversed per unit time.

anhydrous Describing a chemical or other solid substance whose water of crystallisation has been removed.

anisotropic Exhibiting different properties when characteristics are measured along different directions or axes.

annealing Treating metals, alloys or glass by heating and controlled slow cooling, primarily to soften them and remove residual internal stress.

annular nozzle A nozzle whose inlet opening is ring shaped rather than an open circle.

annunciator A device or group of devices that call attention to changes in process conditions that have occurred. Usually included are sequence logic circuits, labeled visual displays, audible devices, and manually operated acknowldege and reset push buttons.

antialias filter A low-pass filter designed to block frequencies greater than one-half the measuring rate.

anti cavitation trim A combination of plug and seat ring or plug and cage that by its geometry permits noncavitating operation or reduces the tendency to cavitate, thereby minimising damage to the valve parts, and the downstream piping.

anti noise trim A combination of plug and seat ring or plug and cage that by its geometry reduces the noise generated by fluid flowing through the valve.

anti reset windup Device or circuit that prevents the saturation of the integral mode of a controller that develops during times when control cannot be achieved. Helps to prevent the controlled variable from overshooting its set point when the obstacle to control is removed.

anti surge control Control by which the unstable operating mode of compressors known as "surge" is avoided.

aperiodically damped Reaching a constant value or steady state of change without introducing oscillation.

apparent density The density of loose or compacted particulate matter determined by dividing actual weight by volume occupied; apparent density is always less than true density of a material comprising the particulate matter because volume occupied includes the space devoted to pores or cavities between particles.

armoured meter tube Variable area meter tube (rotameter) of all metal construction utilising magnetic coupling between the float and an external follower.

as built A document revision that includes all mdifications performed as a result of actual fabrication or installation.

ASCII A widely used code (American Standard Code for Information Interchange) in which alphanumerics, punctuation marks, and certain special machine characters are represented by unique, 7 bit, binary numbers; 128 different binary combinations are possible (27 = 128), thus 128 characters may be represented.

ASCII file A text file that uses only the ASCII character set.

ASIC Application Specific Integrated Circuit.

aspirating burner A burner in which the fuel in a gaseous or finely divided form is burned in suspension, the air for combustion being supplied by bringing into contact with the fuel, air drawn through one or more openings by the lower static pressure created by the velocity of the fuel stream.

aspiration Using a vacuum to draw up gas or granular material, often by passing a stream of water across the end of an open tube, or through the run of a tee joint, where the open tube or branch pipe extends into a reservoir containing the gas or granular material.

asynchronous Circuitry or operation without common clock or timing signals.

asynchronous communication Often called start/stop transmission, a way of transmitting data in which each character is preceded by a start bit and followed by a stop bit.

asynchronous transmission Transmission in which information character, or sometimes each word or small block, is individually synchronised, usually by the use of start and stop elements. The gap between each character (or word) is not of a necessarily fixed length.

atmospheric pressure The barometric reading of pressure exerted by the atmosphere. At sea level 14.7 lb per sq in. or 29.92 in. of mercury.

atomic mass unit A unit for expressing atomic weights and other small masses; it equals, exactly, 1/12 the mass of the carbon 12 nuclide.

atomic number An integer that designates the position of an element in the periodic table of the elements; it equals the number of protons in the nucleus and the number of electrons in the electrically neutral atom.

atomic weight The weight of a single atom of any given chemical element; it is usually taken as the weighted average of the weights of the naturally occurring nuclides, expressed in atomic mass units.

attemperation Regulating the temperature of a substance eg passing superheated steam through a heat exchanger or injecting water mist into it to regulate final steam temperature.

attenuation The reciprocal of gain, when the gain is less than one. It may be expressed as a "dimensionless ratio," "scalar ratio," or in decibels as "20 times the log 10 of that ratio".

audio Pertaining to audible sound-usually taken as sound frequencies in the range

20 to 20,000 Hz.

austenitic stainless steel An alloy of iron containing at least 12% Cr plus sufficient Ni (or in some specialty stainless steels, Mn) to stabilise the face centreed cubic crystal structure of iron at room temperature.

auto-tuning Controller feature that calculates PID settings based on calculations using measured process dynamics and combining those with the parameters of a PID controller. Calculations may be based on transient responses, frequency responses or parametric models.

autoclave An airtight vessel for heating its contents and sometimes agitating them; it usually uses high pressure steam to perform processing, sterilising or cooking steps using moist or dry heat.

autoignition temperature (AFT) The minimum uniform temperature required to initiate or cause self sustained combustion of solid, liquid, or gaseous substance independent of any other ignition source.

autotransformer A type of transformer in which portions of the windings are shared by the primary and secondary circuits.

availability The number of hours in the reporting period less the total downtime for the reporting period divided by the number of hours in the reporting period (expressed in percent).

available energy Energy that theoretically can be converted to mechanical power.

available heat In a thermodynamic working fluid, the amount of heat that could be transformed into mechanical work under ideal conditions by reducing the temperature of the working fluid to the lowest temperature available for heat discard.

available power An attribute of a linear source of electric power defined as Vrms/4R, where V rms is the open circuit rms voltage of the power source and R is the resistive component of the internal impedance of the power source.

available power gain An attribute of a linear transducer defined as the ratio of power available from the output terminals of the transducer to the power available from the input circuit under specified conditions of input termination.

available work The capacity of a fluid or body to do work if applied to an ideal engine.

average position action A type of control system action in which the final control element is positioned in either of two fixed positions, the average time at each position being determined from some function of the measured value of the controlled variable.

averaging pitot tube An adaptation of the pitot tube in which a multiple ported pitot tube spans the process tube; total pressure is measured as a composite of the pressures on several ports facing upstream while static pressure is measured using one or more ports facing downstream.

axial fan Consists of a propeller or disc type of wheel within a cylinder discharging the air parallel to the axis of the wheel.

axial flow Describing a machine such as a pump or compressor in which the general direction of fluid flow is parallel to the axis of its rotating shaft.

azeotrope A mixture whose evolved vapour composition is the same as the liquid it comes from. This phenomenon occurs at one fixed composition for a given system. At either side of the azeotropic point, the vapours will have different compositions from that of the liquid they evolved from. Such mixtures act as pure substances in distillation and thus are inseparable by standard distillation methods. Azeotropic distillation is necessary to separate such a mixture.

azeotropic distillation A distillation technique in which one of the product streams is an azeotrope. It is sometimes used to separate two components by adding a third, which forms an azeotrope with one of the original two components.

back pressure The absolute pressure level as measured four pipe diameters downstream from the turbine flowmeter under operating conditions, expressed in pascals

baffle A plate or vane, plain or perforated, used to regulate or direct the flow of fluid.

balanced trim An arrangement of ports and plug or combination of plug, cage, seals and ports that tends to equalise the pressure above and below the valve plug to minimise the net static and dynamic fluid flow forces acting along the axis of the stem of a globe valve.

balanced draft A system of furnace pressure control in which the inlet air flow or the outlet flue gas flow is controlled to maintain the furnace pressure at a fixed value (typically slightly below atmospheric).

ball check valve A valve that permits flow in one direction only by lifting a spring loaded ball off its seat when a pressure differential acts in that direction and by forcing the ball more tightly against the seat when a pressure differential acts in the opposite flow direction.

ball type viscometer An apparatus for determining viscosity, especially of high viscosity oils and other fluids, in which the time required for a ball to fall through liquid confined in a tube is measured.

ball valve A valve which modifies flow rates with rotary motion of the closure member, which is either a sphere with an internal passage or a segment of a spherical surface.

bandwidth The difference, expressed in hertz, between the two boundaries of a frequency range.

bang-bang control The same as "two-position control."

Barkometer scale A specific gravity scale used primarily in the tanning industry, in which specific gravity of a water solution is determined from the formula: sp gr=1.000 + 0.001n where n is degrees Barkometer; on this scale, water has a specific gravity of zero Barkometer.

barometric pressure Atmospheric pressure as determined by a barometer usually expressed in inches of mercury.

barrel A unit of volume; for petroleum, it equals 9702 cubic inches

barrier Physical entity that limits current and voltage into a hazardous area in order to satisfy Intrinsic Safety requirements.

base 1. The fundamental number of characters available for use in each digital position in a numbering system. 2. A chemical substance that hydrolyses to yield OH- ions. 3. A number that is multiplied by itself as many times as indicated by an exponent.

baseband Communications signals whose bandwidth is approximately equal to the highest frequency transmitted. Compare with modulated signals (RF) where the frequencies transmitted (at the modulated carrier frequency) are much higher than the bandwidth.

base load The term applied to that portion of a station or boiler load that is practically constant for long periods.

basis weight For paper and certain other sheet products, the weight per unit area.

batch 1. The quantity of material required for or produced by a production operation at a single time. 2. An amount of material that undergoes some unit chemical process or physical mixing operation to make the final product homogeneous or uniform. 3. A group of similar computer transactions joined together for processing as a single unit.

batch control (control, batch) Control system that controls a batch process, that is, a process that due to physical structuring of the process equipment or other factors, consists of a sequence of one or more steps or phases that must be performed in a defined order.

batch distillation A distillation process in which a fixed amount of a mixture is charged, followed by an increase in temperature to boil off the volatile components. This process differs from continuous distillation, in which the feed is charged continuously.

batch process A process that manufactures a finite quantity of material by subjecting measured quantities of raw materials to a time sequential order of processing actions using one or more pieces of equipment.

batch processing The technique of executing a set of programs such that each is completed before the next program of the set is started.

baud The measure of the rate at which digital data is transmitted, in bits per second.

Baume scale Either of two specific gravity scales devised by French chemist Antoine Baume in 1768 and often used to express the specific gravity of acids, syrups and other liquids; for light liquids the scale is determined from the formula: ° Be = (140/sp. gr.) - 130. For heavy liquids it is determined from: °Be = 145 (145/sp. gr.). 60°F is the standard temperature used.

Bernoulli coefficient In any stream, if the area is changed, as by a reducer, there is a change in the velocity and a corresponding change in the static pressure, or "head. " This pressure change is measured in units of velocity head. The dimensionless coefficient used for this purpose is the Bernoulli coefficient.

bessel The filter characteristic in which phase linearity across the pass band, rather than amplitude linearity, is emphasised; known also as "constant delay."

best straight line A line midway between the two parallel straight lines closest together and enclosing all output vs. measurand values on a calibration curve.

bimetallic thermometer element A temperature sensitive strip of metal (or other configuration) made by bonding or mechanically joining two dissimilar strips of metal together in such a manner that small changes in temperature will cause the composite assembly to distort elastically, and produce a predictable deflection; the element is designed to take advantage of the fact that different metals have different coefficients of thermal expansion.

binary A term applied to a signal or device that has only two discrete positions or states.

binary code A code that uses two distinct characters, usually 0 and 1.

binary coded decimal (BCD) Describing a decimal notation in which the individual decimal digits are represented by a group of binary bits, e.g., in the 8 4 2 1 coded decimal notation each decimal digit is represented by a group of four binary bits.

binary distillation A distillation process that separates only two components.

Bingham body A non Newtonian substance that exhibits true plastic behavior that is, it flows when subjected to a continually increasing shear stress only after a definite yield point has been exceeded. biphase A method of bit encoding for serial data transmission or recording whereby there is a signal transition every bit period.

bit An abbreviation of binary digit

bit error rate The ratio of bits received in error to bits sent.

bit error rate tester (BERT) A system which measures the fraction of bits transmitted incorrectly by a digital communication system.

bits per second In a serial transmission, the instantaneous bit speed within one character, as transmitted by a machine or a channel. See "baud. "

bituminous Describing a substance that contains organic matter, mostly in the form of tarry hydrocarbons.

black liquor The solution remaining after cooking pulpwood in the soda or sulfite papermaking process.

blade type consistency sensor A pneumatic device for determining changes in consistency of a flowing non Newtonian substance such as a slurry; it senses the force required for a shaped blade to shear through the flowing stock, and transmits an output signal proportional to changes in consistency; its normal operating range is 1.75 to 6.0% suspended solids, with a sensitivity of-0.02% in many

blowdown 1. In a safety valve, the difference between opening and closing pressures. 2. In a steam boiler, the practice of periodically opening valves attached to the bottom of steam drums and water drums, during boiler operation, to drain off accumulations of sediment.

Bode diagram In process instrumentation, a plot of log gain (magnitude ratio) and phase angle values on a log frequency base for a transfer function.

body The part of the valve which is the main pressure boundary. The body also provides the pipe connecting ends, the fluid flow passageway, and may support the seating surfaces and the valve closure member .

body, split A valve body design in which trim is secured between two segments of a

valve body.

body, wafer A thin annular section body whose end surfaces are located and clamped

between the piping flanges by bolts extending from flange to flange.

body, wafer, lugged A thin annular section body whose end surfaces mount between the pipeline

flanges, or may be attached to the end of a pipeline without any additional flange or retaining parts, using either through bolting and/or tapped holes.

body, weir type A body having a raised contour contacted by a diaphragm to shut off fluid flow.

boiler drum level Watertube boiler process variable that is measured and controlled by adding feedwater. Control may be single element, two-element or three-element control. Control strategy will depend on user requirements and the boiler design. Measured variables may include drum level, steam flow, feedwater flow, blowdown flow, drum pressure and feedwater pressure.

boilup Vapours that are generated in the column reboiler.

bomb calorimeter An apparatus for measuring the quantity of heat released by a chemical

reaction; it consists of a strong walled metal container (bomb) immersed in

about 2.5 liters of water in an insulated container; a sample is sealed in the

bomb, the bomb immersed, the reaction started by remote control, and the heat released measured by observing the rise in temperature of the water bath.

bone dry A papermaking term used to describe pulp fibers or paper from which all

water has been removed. Also known as "oven dry"; is moisture free."

bonnet That portion of the valve pressure retaining boundary which may guide the stem and contains the packing box and stem seal. It may also provide the principal opening to the body cavity for assembly of internal parts or be an integral part of the valve body. It may also provide the attachment of the actuator to the valve body

Boolean algebra A process of reasoning, or a deductive system of theorems using a symbolic logic, and dealing with classes, propositions, or on offcircuit elements. It employs symbols to represent operators such as and, or, not, except, if, then, etc., to permit mathematical calculation. Named after English mathematician George Boole.

booster relay A volume or pressure amplifying pneumatic relay that is used to reduce the time lag in pneumatic circuits by reproducing pneumatic signals with high volume and/or high pressure outputs.

bottoms The higher boiling product streams usually taken from the bottom of a distillation column sometimes from the reboiler and sometimes from a separate surge vessel.

boundary layer In a flowing fluid, a low velocity region along a tube wall or other boundary surface.

bound water In a moist solid to be dried, that portion of the water content which is chemically combined with the solid matter.

Bourdon tube A pressure sensing element consisting of a twisted or curved tube of noncircular cross section which tends to be straightened by the application of internal pressure. Also known as "Bourdon element" when used in a "Bourdon pressure gauge."

Brix scale A specific gravity scale used in sugar refining; the degrees Brix represent the weight percent pure sucrose in water solution at 17.5°C.

broadband A communication medium based on CATV technology where multiple signals are frequency division multiplexed.

broadband pyrometer See "wideband radiation thermometer."

broadcast A message addressed to all stations connected to a LAN.

Buna N A nitrile synthetic rubber known for resistance to oils and solvents.

buoyancy The tendency of a fluid to lift any object submerged in the body of the fluid; the amount of force applied to the body equals the product of fluid density and volume of fluid displaced.

buoyancy displacer The technique of measuring liquid level by measuring the buoyant force on a partially immersed volumetric displacing device (displacer).

burn-in Device operation, usually under accelerated environmental conditions that simulate life in the devices' intended application, used to detect early-life (infantile) failures.

bus A group of wires or conductors, considered as a single entity, which interconnects part of a system.

butterfly valve A valve consisting of a disc inside a valve body which operates by rotating about an axis in the plane of the disc to shut off or regulate flow; a similar device used in heating or ventilating ductwork is called a butterfly damper.

Butterworth The filter characteristic in which constant amplitude across the pass band is the objective; known also as "constant amplitude (CA)."

byte A sequence of adjacent binary digits (bits) operated upon as a unit and usually shorter than a word, commonly an eight bit segment of a computer word. A byte can be used to store one ASCII character.

Cage A part in a globe valve surrounding the closure member to provide alignment and facilitate assembly of other parts of the valve trim. The cage may also provide flow characterisation and/or a seating surface for globe valves and flow characterisation for some plug valves

cage guide A valve plug fitted to the inside diameter of the cage to align the plug with the seat.

calibration curve A plot of indicated value versus true value used to adjust instrument readings for inherent error; a calibration curve is usually determined for each calibrated instrument in a standard procedure and its validity confirmed or a new calibration curve determined by periodically repeating the procedure.

calibration gas A gas with known concentrations of components that is used as a standard for calibration of analysers or gas detector alarm level(s).

calibration traceability The relationship of the calibration of an instrument through a step by step process to an instrument or group of instruments calibrated and certified by a national standardising laboratory.

calibration uncertainty The maximum calculated error in the output values, shown in a calibration record, due to causes not attributable to the transducer.

Calorie The mean calorie is 1/100 of the heat required to raise the temperature of 1 gram of water from 0°C to 100°C at a constant atmospheric pressure. Also defined as 3600/860 joules, a joule being the amount of heat produced by a watt in one second.

calorific value The number of heat units liberated per unit of quantity of a fuel burned in a calorimeter under prescribed conditions.

Calorimeter 1. A device for determining the amount of heat liberated during a chemical reaction, change of state or dissolution process. 2. Apparatus for determining the calorific value of a fuel.

Candela Metric unit for luminous intensity. The unit used to express the intensity of light visible to the human eye. It corresponds to the emission from 1/60th of a square centimeter of a black body operating at the solidification temperature of platinum, and emitting one lumen per steradian.

Capacitance The ability of a capacitor to store a charge. The greater the capacitance, the greater the charge that can be stored.

Capacitor A device used for storing an electrical charge.

Capacity 1. The rate of flow through a valve under stated test conditions. 2. A measure of the maximum quantity of energy or material which can be stored within a given piece of equipment or system.

capacity lag In any process, the amount of time it takes to supply energy or material to a

storage element at one point in the process from a storage point elsewhere in

the process. Also known as "transfer lag. "

capillary 1. Having a very small internal diameter. 2. A tube with a very small diameter.

capillary action 1. Spontaneous elevation or depression of a liquid level in a fine hair like tube when it is dipped into a body of the liquid. 2. Capillary action is induced by differences in surface energy between the liquid and the tube material.

capillary tube A tube sufficiently fine that capillary action is significant.

capsule A pressure sensing element consisting of two metallic diaphragms joined around their peripheries.

carrier detect Modem interface signal defined by the RS-232 standard that indicates to a receiving device that a signal is coming from a distant modem.

carrier frequency The basic frequency or pulse repetition rate of a transmitted signal, bearing no intrinsic intelligence until it is modulated by another signal that does bear intelligence.

carrier sense multiple access with collision detect (CSMA/CD) A network access procedure where a device with data to transmit first listens to the medium. When the medium is not busy, the device starts transmitting. While the device is transmitting, it listens for collisions (simultaneous transmission by another station). If a collision occurs, the node stops transmitting, waits, and tries again.

cascade A control system composed of two loops where the set point of one loop (the inner loop) is the output of the controller of the other loop (the outer loop).

catalysis A phenomenon in which a relatively small amount of substance (catalyst) augments the

rate of a chemical reaction without itself being consumed.

catalytic cracking Conversion of high-boiling hydrocarbons into lower-boiling hydrocarbons by

a catalyst.

CATV Community Antenna Television. See "broadband.

caustic soda sodium hydroxide that contains 76 to 78% sodium oxide.

cavitation A two stage phenomenon of liquid flow. The first stage is the formation of voids or cavities within the liquid system; the second stage is the collapse or implosion of these cavities back into an all liquid state.

cavitation erosion Progressive removal of surface material due to localised hydrodynamic impact forces associated with the formation and subsequent collapse of bubbles in a liquid in contact with the damaged surface. Also known as "cavitation damage"; "liquid erosion failure."

CCITT The Comite Consultatif Internationale de Telegraphie et Telephonie, an international consultative committee that sets standards for voice and data communications .

CCITT V.xx

International standards in communications concerned with modem interfaces,

speeds, and transmission modes (i.e., V.22). CCITT X.25 International standard defining the protocol used in access to a packet switching network.

CD ROM A compact disk used for computer data storage. The letters stand for "Compact Disk Read Only Memory. "

Celsius A scale for temperature measurement based on the definition of 0°C and 100°

C as the freezing point and boiling point, respectively, of pure water at standard pressure.

centipoise (cp) A unit of viscosity which is equal to 0.01 poise. centistoke (cs) A cgs unit of kinematic viscosity in customary use, equal to the kinematic viscosity of a fluid having a dynamic viscosity of 1 centipoise and a density of 1 gram per cubic centimeter.

central processing unit (CPU) The part of a computing system that contains the arithmetic and logical units, instruction control unit, timing generators, and memory and I/O interfaces.

centrifugal compressor A machine in which a gas or vapour is compressed by radial acceleration in an impeller with a surrounding casing and can be multistaged to achieve high ratios of compression. centrifugal fan Consists of a fan rotor or wheel within a housing discharging the air at right

angle to the axis of the wheel.

centrifugal force A force acting in a direction along andoutward on the radius of turn for a

mass in motion.

centrifugal pump A machine for moving a liquid by accelerating it radially outward in an

impeller to a surrounding volute casing.

characteristic curve 1. A graph (curve) which shows the ideal values at steady state or an output variable of a system as a function of an input variable, the other input variables being maintained at specified constant values.

characteristic, equal percentage The inherent flow characteristic which, for equal increments of rated travel, will ideally give equal percentage changes of the existing flow coefficient (Cv). characteristic, inherent flow The relationship between the flow rate through a valve and the travel of the closure member as the closure member is moved from the closed position to rated travel with constant pressure drop across the valve.

characteristic, installed flow The relationship between the flow rate through a valve and the travel of the closure member as the closure member is moved from the closed position to rated travel when the pressure drop across the valve varies as influenced by the system in which the valve is installed. characteristic, linear flow An inherent flow characteristic which can be represented by a straight line on a rectangular plot of flow coefficient (Cv) versus percent rated travel. Therefore, equal increments of travel provide equal increments of flow coefficient (Cv) at constant pressure drop.

characteristic, modified parabolic flow An inherent flow characteristic which provides fine throttling action at low valve plug travel and approximately a linear characteristic for upper portions of valve travel. It is approximately midway between linear and equal percentage.

characteristic, quick-opening flow An inherent flow characteristic in which there is a maximum flow with minimum travel.

characterised cam A component in a valve positioner used to relate the closure component position to the control signal.

characterised sleeve A part added to a plug valve to provide various flow characteristics. characterised trim Control valve trim that provides a predefined flow characteristic.

checksum Entry at the end of a block of data corresponding to the binary sum of all information in the block. Used in error checking procedures.

check valve A flow control device that permits flow in one direction and prevents flow in the opposite direction.

choke coil An inductor that allows direct current to pass but presents relatively large impedance to alternating current.

choked flow The condition that exists when, with the upstream conditions remaining constant, the flow through a valve cannot be further increased by lowering the downstream pressure. chromatogram The pattern formed by the chromatograph output represents zones of separated elements and compounds on a strip chart. The time the peak appears and the area under the peak identify the component and the concentration.

chromatograph Analytical instrument that uses chromatography to separate substances to analyse for chemical composition and concentration.

chromatography Procedure for separating components from a mixture of chemical substances which depends on selective retardation and physical absorption of substances by a porous bed of sorptive media as the substances are transported through the bed by a moving fluid; the sorptive bed (stationary phase) may be a solid or a liquid dispersed on a porous, inert solid; the moving fluid (moving phase) may be a liquid solution of the substances or a mixture of a carrier gas and the vapourised sample; various detection techniques are used, some of which can be automated.

Cipolletti weir An open channel flow measurement device similar to a rectangular weir but having sloping sides, which results in a simplified discharge equation.

circuit breaker A device designed to allow manual opening and closing of a circuit and also to open the circuit automatically on a predetermined overload of current without damage to itself.

classification of a location The assignment of a rating such as Division 1, Division 2, or nonhazardous.

clear-to-send (CTS) Modem interface signal defined by the U.S. standard EIA-RS-232-C that indicates to data terminal equipment that it may begin data transmission.

clipboard In data processing, an area of information can be stored in order to use it later in a different application.

closed loop A combination of control units in which the process variable is measured and compared with the desired value (or set point). If the measured value differs from the desired value, a corrective signal is sent to the final control element to bring the controlled variable as close as possible to the desired value.

closed-loop system A system with a feedback type of control, such that the output is used to modify the input.

coaxial cable Cable with a centre conductor surrounded by a dielectric sheath and an external conductor. Has controlled impedance characteristics that make it valuable for data transmission. CODEC A device which consists of an encoder (which translates an analogue signal to a digital code) and a decoder (which performs the reverse operation).

coefficient, flow A constant (Cv), related to the geometry of a valve, for a given valve opening, that can be used to predict flow rate.

coefficient of discharge The ratio of actual flow to theoretical flow. It includes the effects of jet contraction and turbulence.

coefficient, rated flow The flow coefficient (Cv) of the valve at rated travel.

coefficient, relative flow The ratio of the flow coefficient (Cy) at a stated travel to the flow coefficient (Cv) at rated travel.

coefficient, valve recovery See "liquid pressure recovery factor".

cold junction See " reference junction".

combustible dusts Dusts which (when mixed with air in certain proportions) can be ignited and will propagate flame.

combustible gas Any flammable or combustible gas or vapour (but not atomised liquid) that can, in sufficient concentration by volume in air, become the fuel for combustion.

combustion (flame) safeguard A system for sensing the presence or absence of flame and indicating, alarming or initiating control action.

COM file A computer file name ending in .COM which most often contains a machine code program. It is short for " command " file.

common A reference within a system having same electrical potential throughout. Usually connected to ground at a single point. Often multiple commons are used throughout a system such as power common and signal.

common carrier Company that furnishes communications services to the general public.

common mode In analogue data, an interfering voltage from both sides of a differential input pair (in common) to ground.

common mode interference A form of interference which appears between the terminals of any measuring circuit and ground.

common mode rejection (CMR) The ability of circuit to discriminate against a common mode voltage. NOTE: It may be expressed as a dimensionless ratio, a scalar ratio, or in decibels as 20 times the log10 of that ratio.

common mode rejection ratio (CMRR). A measure of the ability of a detector to damp out the effect of a common-mode-generated interference voltage; usually expressed in decibels.

common mode voltage In-phase, equal-amplitude signals that are applied to both inputs of a differential amplifier, usually referred to as a guard shield or chassis ground.

communications protocol The rules governing the orderly exchange of information between devices on a data link.

compensation Provision of a supplemental device, circuit, or special materials to counteract known sources of error.

complementary metal oxide semiconductor (CMOS) 1. One type of computer semiconductor memory. The main feature of CMOS memory is low power consumption. 2. A type of semiconductor device not specifically memory.

compliance The reciprocal of stiffness.

compressibility Volumetric strain per unit change in hydrostatic pressure.

compressibility factor (Z) A factor used to compensate for deviation from the laws of perfect gases. If the gas laws are used to compute the specific weight of a gas, the computed value must be adjusted by the compressibility factor Z to obtain the true specific weight.

compressible flow Fluid flow under conditions which cause significant changes in density.

compressor A machine-usually a reciprocating-piston, centrifugal/ or axial-flow designchich is used to increase pressure in a gas or vapour.

computing relay A device that performs one or more calculations or logical functions or both, and sends out one or more resultant signals.

concatenate To combine several files into one file, or several strings of characters into one string, by appending one file or string after another.

concentric orifice plate A fluid-meter orifice plate having a circular opening whose centre coincides with the axis of the centre of the pipe in which it is installed.

Condensate 1. The liquid product of a condensing cycle. 2. A light hydrocarbon mixture formed by expanding and cooling gas in a gas-recycling plant to produce a liquid output.

condensate pot A section of pipe (100mm. diameter) typically installed horisontally at the orifice flange union to provide a large-area surge surface for movement of the impulse line fluid with instrument element position change to reduce measurement error from hydrostatic head difference in the impulse lines.

condensate trap 1. A device to separate saturated water from steam in a pipe or piece of process equipment. 2. A device used to trap and retain condensate in a measurement impulse line to prevent hot vapours from reaching the instrument.

condensation-type hygrometer A dew point instruments tha operates by detecting the equilibrium temperature at which dew or frost forms on a thermoelectrically, mechanically or chemically cooled surface.

condenser The heat exchanger, located at the top of the column, that condenses overhead vapours. For distillation, the common condenser cooling media are water, air, and refrigerants such as propane. The condenser may be partial or total. In a partial condenser only part of the vapours are condensed, with the remainder usually withdrawn as a vapour product.

conditional stability A linear system is conditionally stable if it is stable for a certain interval of values of the open-loop gain, and unstable for certain lower and higher values.

condition monitoring system A system designed to monitor the condition of a machine or process.

conduction The transmission of heat through and by means of matter unaccompanied by any obvious motion in the matter.

conductivity (thermal) The amount of heat (Btu) transmitted in one hour through one square foot of a homogeneous material 1 in. thick for a difference in temperature of 1° F between the two surfaces of the material.

conductivity (electrical) The electrical conductance, at a specified temperature, between the opposite faces of a unit cube; usually expressed as ohm-l cm-1.

conductor Any material through which electrical current can flow.

conduit (liquid) 1. Any channel, duct, pipe or tube for transmitting fluid along a defined flow path.

conduit (electrical) thin-wall pipe used to enclose wiring.

cone-plate viscometer An instrument for determining the absolute viscosity of fluids in small sample volumes by sensing the resistance to rotation of a moving cone caused by the presence of the test fluid in a space between the cone and a stationary flat plate.

CONFIG.SYS An MSDOS computer file that establishes the MSDOS operating system environment and installs memory resident software such as device drivers.

configurable A term applied to a device or system whose functional characteristics can be selected or rearranged through programming or other "soft" methods. Excludes "hard" rewiring as a means of altering the configuration.

confined flow Flow of a continuous stream fluid within a process vessel or conduit;

conical orifice An orifice having a 45 bevel on the inlet edge to yield more constant and predictable discharge coefficient at low flow velocity (Reynolds number less than 10,000).

conservation of charge The principle that states the total charge of an isolated system is constant. Also known as "charge conservation. "

conservation of energy (Newton's law) The principle that energy cannot be created or destroyed, although it can be changed from one form to another. Also known as "energy conservation."

consistency A qualitative means of classifying substances, especially semisolids, according to their resistance to dynamic changes in shape.

constant-head meter A flow measurement device that maintains a constant pressure differential by varying the cross section of a flow path through the meter, such as in a piston meter or rotameter.

constant-volume gas thermometer A device for detecting and indicating temperature based on Charles Law-the pressure of a confined gas varies directly with absolute temperature; in practical instruments, a bulb immersed in the thermal medium is connected to a Bourdon tube by means of a capillary; changes in temperature are indicated directly by movement of the Bourdon tube due to changes in bulb pressure.

consumables Those materials or component are depleted or require periodic replacement through normal use of the instrument.

contactor A mechanical or electromechanical device for making and breaking electrical continuity between two branches of a power circuit, thereby establishing or interrupting current flow.

contention A condition on a multidrop communication channel when two or more locations try to transmit at the same time.

contiguous file A file consisting of physically adjacent blocks on a mass-storage device.

continuous blowdown The uninterrupted removal of concentrated boiler water from a boiler to control total solids concentration in the remaining water

continuous dilution A technique of supplying a protective gas flow continuously to an enclosure, housing electrical circuitry, containing an internal potential source of flammable gas or vapour for the purpose of diluting any flammable gas or vapour which could be present to a level well below the lower explosion limit (LEL).

continuous-duty rating The maximum power or other operating characteristic that a specific device can sustain indefinitely without significant degradation of its functions.

continuous rating The rating applicable to specified operation for a specified uninterrupted length of time.

control action of a controller or a controlling system, the nature of the change of the output effected by the input. See "proportional control action," "integral control action, and " derivative control action. "

control action, derivative (rate) (D) In process instrumentation, control action in which the output is proportional to the rate of change of the inputy.

control action, floating In process instrumentation, control action in which the rate of change of the output variable is a predetermined function of the input variable.

control action, integral (reset) (I) Control action in which the output is proportional to the time integral of the input.

control action, proportional (P) Control action in which there is a continuous linear relation between the output and the input.

control action, proportional-plus-derivative (rate) (PD) Control action in which the output is proportional to a linear combination of the input and the time rate of change of input.

control action, proportional-plus-integral (reset) (PI) Control action in which the output is proportional to linear combination of the input and the time integral of the input.

control action, proportional-plus-integral (reset) -plus-derivative (rate) (PID) Control action in which the output is proportional to a linear combination of the input, the time integral of input and the time rate of change of input.

control, adaptive Control in which automatic means are used to change the type or influence (or both) of control parameters in such a way as to im-prove the performance of the control system

control algorithm A mathematical representation of the control action to be performed.

control, cascade Control in which the output of one controller (primary of the cascade) is the set point for another controller (secondary of the cascade).

Control character A character whose purpose is to control an action rather than to pass data to a program; ASCII control characters have an octal code between O and 37; normally typed by holding down the-CTRL key on a terminal keyboard while striking a character key.

control, differential gap Control in which the output of a controller remains at a maximum or minimum value until the controlled variable crosses a band or gap, causing the output to reverse. The controlled variable must then cross the gap in the opposite direction before the output is restored to its original condition.

control, direct digital Control performed by a digital device which establishes the signal to the final controlling element. control element A component of a control system that reacts to manipulate a process attribute when stimulated by an actuating signal.

control, feedback Control in which a measured variable is compared to its desired value to produce an actuating error signal which is acted upon in such a way as to reduce the magnitude of the error.

control, feedforward Control in which information concerning one or more conditions that can disturb the controlled variable is converted, outside of any feedback loop, into corrective action to minimise deviations of the controlled variable.

control, high-limiting Control in which the output signal is prevented from exceeding a predetermined high limiting value.

controlled medium The process fluid or other substance containing the controlled variable.

controlled variable 1. The variable which the control system attempts to keep at the set point value. The set point may be constant or variable. 2. The part of a process to be controlled (flow, level, temperature, pressure, etc.). 3. A process variable which is to be controlled at some desired value by means of manipulating another process variable.

controller A device or program which operates automatically to regulate a controlled variable. controller, derivative (D) A controller which produces derivative control action only.

controller, direct-acting A controller in which the value of the output signal increases as the value of the input (measured variable) increases See controller, reverse-acting.

controller drift See "drift. "

controller, floating A controller in which the rate of change of the output is a continuous (or at least a piecewise continuous) function of the actuating error signal. The output of the controller may remain at any value in its operating range when the actuating error signal is zero and constant. Hence, the output is said to float.

controller, integral (reset) (I) A controller which produces integral control action only.

controller, multiposition A controller having two or more discrete values of output.

controller, on-off A two-position controller in which one of the two discrete values is zero.

controller, program A controller which automatically holds or changes set point to follow a prescribed program for a process.

controller, proportional (P) A controller which produces proportional control action only. controller, proportional-plus-derivative (rate) (PD) A controller which produces proportional-plus-derivative (rate) control action.

controller, proportional-plus-integral (reset) (PI) A controller which produces proportional-plusintegral (reset) control action.

controller, proportional-plus-integral (reset)plus-derivative (rate) (PID) A controller which produces proportional-plus-integral (reset)-plus-derivative (rate) control action.

controller, ratio A controller which maintains a predetermined ratio between two variables. controller, reverse-acting A controller in which the value of the output signal decreases as the value of the input (measured variable) increases. See controller, direct-acting

controller, sampling A controller using intermittently observed values of a signal to effect control action.

controller, self-operated (regulator) A controller in which all the energy to operate the final controlling element is derived from the controlled system.

control limits In statistical quality control, the upper and lower values of a measured quantity that establish the range of acceptability; if any individual measurement falls outside this range, the part involved is rejected and if the sample average for the same measurement falls outside the range, the entire lot is rejected.

control logic The sequence of steps or events necessary to perform a particular function. Each step or event is defined to be either a single arithmetic or a single Boolean expression.

control loop A combination of two or more instruments or control functions arranged so that signals pass from one to another for the purpose of measurement and/or control of a process variable. See closed loop and open loop.

control loop instability A regular oscillation of a feedback control system caused by excessive loop gain. It is independent of external disturbances.

control, low limiting Control in which output signal is prevented from decreasing beyond a predetermined low limiting value.

control mode A specific type of control action such as proportional, integral, or derivative.

control, optimising Control that automatically seeks and maintains the most advantageous value of a specified variable, rather than maintaining it at one set value.

control, supervisory Control in which the control loops operate independently subject to intermittent corrective action; e.g., set point change from an external source.

control system A system in which deliberate guidance or manipulation is used to achieve a prescribed value of a variable.

control system, automatic A control system which operates without human intervention.

control system, multielement (multivariable) A control system utilising input signals derived from two or more process variables for the purpose of jointly affecting the action of the control system. control system, noninteracting A control system with multiple inputs and outputs in which any given input-output pair is operating independently of any other input-output pair.

control, time-proportioning Control in which the output signal consists of periodic pulses where duration is varied to relate, in some prescribed manner, the time average of the output to the actuating error signal.

control valve gain The change in the flow rate as a function of the change in valve travel.

control, velocity limiting Control in which the rate of change of a specified variable is prevented from exceeding a predetermined limit.

coprocessor A device added to a CPU to perform certain functions (eg floating point operations) more efficiently.

Coriolis effect An accelerating force acting on any body moving freely above the earth's surface. Caused by the rotaion of the earth about its axis. The basis for Coriolis Mass Flow Meters.

Coriolis mass flow meter A mass flow meter which measures mas flow of a fluid by determining the torque resulting from radial acceleration of the fluid.

CPU central processing unit

cracking Thermal decomposition of complex hydrocarbons into simpler components.

cracking furnace Furnace used to produce enough heat to reduce the molecular weight of hydrocarbons by breaking molecular bonds.

critical flow 1. The rate of flow of a fluid equivalent to the speed of sound in that fluid. 2. A point at which the characteristics of flow suffer a finite change. In the case of a liquid, critical flow could mean the point at which the flow regime changes from laminar to transitional or alternatively used to mean the onset of choked flow. In the case of a gas, critical flow may mean the point at which the velocity at the vena contracta attains the velocity of sound or it may mean the point at which the flow is fully choked.

critical point The temperature and pressure which two phases of a substance in equilibrium with each other become identical, forming one phase.

critical pressure 1. The pressure of the liquid-vapour critical point. 2. The equilibrium pressure of a fluid that is at its critical temperature.

critical-pressure ratio The ratio of downstream pressure to upstream pressure which correspond to the onset of turbulent flow in a moving stream of fluid.

critical strain The amount of prior plastic strain that is just sufficient to trigger recrystallisation when a deformed metal is heated.

critical temperature 1. The temperature of the liquid-vapour critical point, that is the temper above which the fluid has no liquid-vapour transition. 2. The temperature of a fluid above which the fluid cannot be liquefied by pressure alone

critical velocity For a given fluid, the average linear velocity marking the upper limit of streamline flow and the lower limit of turbulent flow given temperature and pressure in a given confined flow path.

cross talk The unwanted energy transferred from one circuit, the disturbing circuit, to another circuit, the disturbed circuit. Typically signals electrically coupled from another circuit.

cyclic redundancy check (CRC) An error detection scheme in which a check character is generated from the remainder after dividing all bits in a block of serial data by a predetermined binary number. The CRC is appended to the the transmitted data and recalculated by the receiver to verify transmission accuracy.

Dalton's law States that the total pressure exerted by a mixture of gases equals the sum of the partial pressures that would be exerted if each of the individual gases present were to occupy the same volume by itself.

damped wave A wave in which the source amplitude diminishes with each succeeding cycle.

damper A device for introducing a variable resistance for regulating the volumetric flow of gas or air

damping The progressive reduction or suppression of oscillation in a device or system.

critically damped when the time response is as fast as possible without overshoot,

underdamped when overshoot in the time response occurs

overdamped when the time response is slower than critical

damping factor In any damped oscillation, the ratio of the amplitude of any given half cycle to the amplitude of the succeeding half cycle.

database A collection of interrelated data stored together with controlled relationships. The data are stored so that they are independent of programs that use the data; a common and controlled approach is used in adding new data and in modifying and retrieving existing data within a database.

data compression The elimination of redundant data without loss of information

data highway A communication link between separate stations tied with a multidrop cable or optical connections. It eliminates a need for separate, independently wired data links. Each station on a highway can function independently.

data reduction The process of transforming large quantities of raw data, usually gathered by automatic recording equipment, into useful, condensed, or simplified information.

data set (DS) 1. A collection of data with predefined structure. 2. A device (such as a modem) which performs the modulation/demodulation and control functions necessary to provide compatibility between data processing equipment and communications facilities.

data set ready (DSR) Modem interface signal defined in RS-232 which indicates to the attached terminal equipment that the modem is attached to the telephone circuit.

data signalling rate In communications, the data transmission capacity of a set of parallel channels. The data signaling rate is expressed in bits per second.

data sink In communications, a device capable of accepting data signals from a transmission device. It also may check these signals and originate error control signals.

data, skewed Data sample that has been biased and is not representative.

data source In communications, a device capable of originating data signals for a transmission device. It also may accept error control signals.

data terminal equipment (DTE) 1. The embodiment of the media, modulation, and coding-dependent portion of a fieldbus-connected device, comprising the lower portions of the Physical Layer and all higher layers within the device. 2. The device providing the data source or end point for a transmission link.

data terminal ready (DTR) Modem interface signal defined in RS-232, which indicates to the modem that the terminal equipment is ready for transmission.

data type Any one of several different types of data, such as integer, real, double precision, complex, logical, and Hollerith. Each has a different mathematical or logical significance and may have different internal representation.

data unpacking The process of recovering individual items of data from packed information

datum A value that serves as a reference for measuring other values of the same quantity.

DC voltage A voltage that forces electrons to move through a circuit in the same direction continuously, thereby producing a direct current.

dead band The range through which an input can be varied without initiating an observable response. Dead band is usually expressed in percent of span

dead time The interval of time between initiation of an input change or stimulus and the start ofthe resulting response.

dead time correction A correction applied to an instrument reading to account for events or stimuli occurring during the instrument's dead time.

deadweight gauge A device used to generate accurate pressures for the purpose of calibrating pressure gauges; freely balanced weights (dead weights) are loaded on a calibrated piston to give a static hydraulic pressure output.

deaeration Removing a gas from a liquid or semisolid substance, such as boiler feedwater or food.

debug To submit a newly designed process, mechanism or computer program to simulated or actual operating conditions to detect and eliminate errors and inefficiencies.

decade A group or assembly of ten units, e.g., a counter which counts to ten or a resistor box which inserts resistance quantities in multiples of powers of 10.

decade counter Counter that produces one output pulse for every 10 input pulses.

decanting Boiling or pouring off liquid near the top of a vessel that contains two immiscible liquids or a liquid solid mixture which has separated by sedimentation, without disturbing the heavier liquid or settled solid.

decarburising Removing carbon from the surface layer of a steel or other ferrous alloy by heating it in an atmosphere that reacts selectively with carbon; atmospheres that are relatively rich in water vapour or carbon dioxide are typical deoxidising atmospheres.

decay The spontaneous transformation of a nuclide into one or more other nuclides either by emitting one or more subatomic particles or gamma rays from its nucleus or by nuclear fission.

decay ratio The ratio of the amplitude change of a series of cycles.

decay time The time in which a voltage or current pulse will decrease to one tenth of its maximum value. Decay time is proportional to the time constant of the circuit.

decibel (dB) 1. A unit of level, where: Level in dB = 10 log 10 P1/Pref. P1 = a power, or, quantity directly proportional to power. Pref= a reference power, or, a corresponding reference quantity proportional to power. 2. A measure of magnitude ratio; magnitude ratio is dB = 20 log10 (magnitude ratio).

decision table A table of all contingencies that are to be considered in the description of a problem, together with the actions to be taken. Decision tables can be used in place of flow charts for problem description and documentation.

declaration As used in many programming languages, a statement that is not to be executed, but usually is used for descriptivepurposes.

decoupling The technique of reducing process interaction through coordination of control loops. decoupling control A technique in which interacting control loops are automatically compensated when any one control loop takes a control action.

degree of freedom A number one less than the number of frequencies being tested with a chi-square test.

degree of protection of enclosures An international system of rating standard levels of protection provided by enclosures for the protection provided by an enclosure against ingress of solids and/or liquids. Definitions are found in IEC Publications 529 and 144.

deliquescence The absorption of atmospheric water vapour by a crystalline solid until the crystal dissolves into a saturated solution.

delta network A set of three circuit branches connected in series, end to end, to form a mesh having three nodes.

demodulation The process of retrieving intelligence (data) from a modulated carrier wave. The reverse of "modulation. "

densimeter An instrument for determining the density of a substance in absolute units, or for determining its specific gravity (ie, its relative density with respect to that of pure water). Also known as "density gauge"; "density indicator"; "gravitometer. "

densitometer An instrument for determining optical density of photographic or radiographic film by measuring the intensity of transmitted or reflected light.

density 1. The mass of a unit volume of a liquid at a specified temperature. The units shall be stated, such as kilograms per meter3. 2. Closeness of texture or consistency. 5. Degree of opacity, often referred to as "optical density".

. density correction Any correction made to an instrument reading to compensate for the deviation of density from a fixed reference value; it may be applied because the fluid being measured is not at standard temperature and pressure, because ambient temperature affects density of the fluid in a fluid filled instrument, or because of other similar effects.

density transmitter An instrument used to determine liquid density by measuring the buoyant force on an air filled float immersed in a flowing liquid stream.

derivative This control action will cause the output signal to change according to the rate at which input signal variations occur during a certain time interval.

derivative action A type of control system action in which a predetermined relation exists between the position of the final control element and the derivative of the controlled variable with respect to time.

derivative control Change in the output that is proportional to the rate of change of the input. Also called "rate control."

derivative time The time interval by which rate action advances the effect of proportional action on the final control element.

describing function For a nonlinear element in sinusoidal steady state, the frequency response obtained by taking only the fundamental component of the output signal. The describing function depends on the frequency and on the amplitude of the input signal, or only on the amplitude of the input signal.

design pressure The maximum allowable working pressure permitted under the rules of the relevant Construction Code.

desorption Removing adsorbed material.

desuperheater Equipment used to remove superheat from steam, usually by the injection of water.

detergent A natural material or synthetic substance having the soaplike quality of being able to emulsify oil and remove soil from a surface.

developed boiler horsepower The boiler horsepower generated by a steam generating unit.

deviation A departure from a desired value or expected value or pattern.

dewars Insulated thermos like containers for cryogenic liquids, which can be designed to house detectors or lasers requiring cooling.

dewatering Removing water from solid or semisolid material (for instance, by centrifuging, filtering, settling or evaporation).

dew cell An instrument consisting of two bare electrical wires wound spirally around an electrical insulator and covered by wicking wetted with an aqueous solution containing an excess of LiCI; dew point of the surrounding atmosphere is determined by passing an electric current between the two wires, which raises the temperature of the LiCI solution until its vapour pressure is the same as that of the ambient atmosphere.

dew point The temperature, referred to a specific pressure, at which water vapours condense.

diamagnetic material A substance whose specific permeability is less than 1.00 and is therefore weakly repelled by a magnetic field.

diamond pyramid hardness A material hardness determined by indenting a specimen with a diamond pyramid indenter having a 136° angle between opposite faces then calculating a hardness number by dividing the indenting load by the pyramidal area of the impression. Also known as "Vickers hardness. "

diaphragm A sensing element consisting of a thin, usually circular, plate which is deformed by pressure differential applied across the plate.

diaphragm seal A thin flexible sheet of material clamped between two body halves to form a physical barrier between the instrument and process fluid.

diaphragm valve A valve with a flexible linear motion closure member that is forced into the internal flow passageway of the body by the actuator

dibit A grouping of two bits (i.e., 00, 01, 10, 11).

dichroic filter A filter which selectively transmits some wavelengths of light and reflects others. Typically such filters are based on multilayer interference coatings.

dielectric An insulating material, or a material that can sustain an electric field with very little dissipation of power.

dielectric absorption The persistence of electric polarisation in certain dielectrics after the discharge of a capacitor.

dielectric coating An optical coating made up of one or more layers of dielectric (nonconductive) materials. The layer structure determines what fractions of incident light at various wave lengths are transmitted and reflected.

dielectric constant A material characteristic expressed as the capacitance between two plates when the intervening space is filled with a given insulating material divided by the capacitance of the same plate arrangement when the space is filled with air or is evacuated.

difference equation An equation expressing a functional relationship of one or more independent variables, one or more functions dependent on these variables, and successive differences of these functions.

differential amplifier A device which compares two input signals and amplifies the difference between them.

differential gap The smallest increment of change in a controlled variable required to cause the final control element in a two position control system to move from one position to its alternative position.

differential input The difference between the instantaneous values of two voltages both being biased by a common mode voltage.

differential pressure 1. The difference in pressure between two points of measurement. 2. The static pressure difference generated by the primary device when there is no difference in elevation between the upstream and downstream pressure taps.

differential pressure gauge An instrument designed to measure the difference in pressure between two enclosed spaces, independent of their absolute pressures.

differential pressure transmitter A transducer designed to measure the pressure difference between two points in a process and transmit a signal proportional to this difference, without regard to the absolute pressure at either point.

differential pressure type liquid level meter A device designed to measure the head of liquid in a tank above some minimum level and produce an indication proportional to this value.

differentiation The act of taking a derivative. Converts displacement to velocity and velocity to acceleration.

differentiator A device whose output function is proportional to the derivative, i.e., the rate of change, of its input function with respect to one or more variables (usually with respect to time).

diffraction A phenomenon associated with the scattering of waves when they encounter obstacles whose size is about the same order of magnitude as the wavelength. Diffraction forms the basis for x ray crystallography, and also tends to produce aberrations that must be accomodated in the design and construction of high quality acoustical and optical systems.

diffraction grating An array of fine, parallel, equally spaced reflecting or transmitting lines which diffract light into a direction characteristic of the spacing of the lines and the wavelength of the diffracted light.

diffuser A duct, chamber or enclosure in which low pressure, high velocity flow of a fluid, usually air, is converted to high pressure, low velocity flow.

diffusion Migration of atoms, molecules or ions spontaneously, under the driving force of compositional differences, and using only the energy of thermal excitation to cause atom movements.

digital filter An algorithm which reduces undesirable frequencies in the signal.

digital input A number value or binary (two state) input. See "input, digital."

digital logic A signal level is represented as a number value with a most significant and least significant bit. Binary digital logic uses numbers consisting of strings of ls and 0s.

digital output Transducer output that represents the magnitude of the measurand in the form of a series of discrete quantities coded in a system of notation.

digital signal A signal that has only two (on or off, 1 or 0) possible values. 2. A discrete or discontinuous signal, one whose various states are discrete intervals apart.

digital to analogue converter (D/A or DAC) A device, or subsystem that converts binary (digital) data into continuous analogue data, as, for example, to drive actuators of various types, motor speed controllers, etc.

digital valve A single valve casing containing multiple solenoid valves whose flow capacities vary in binary sequence (1, 2, 4, 8, 16, ...); to regulate flow, the control device sends operating signals to various combinations of the solenoids; applications are limited to very clean fluids at moderate temperatures and pressures.

digitise To convert an analogue measurement of a physical variable into a numerical value, thereby expressing the quantity in digital form. see "analogue to digital converter. "

dilatometer An apparatus for accurately measuring thermal expansion of materials.

dilution Adding solvent to a solution to lower its concentration.

DIN Abbreviation for the standards institution of the Federal Republic of Germany.

diode A two electrode electronic component containing merely an anode and a cathode.

diode laser A laser in which stimulated emission is produced at a p n junction in a semiconductor material. Only certain materials are suited for diode laser operation, among them gallium arsenide, indium phosphide, and certain lead salts.

diopter A measurement of refractive power of a lens equal to the reciprocal of the focal length in meters. A lens with 20 centimeter focal length has power of five diopters, while one with a 2 meter focal length has a power of 0.5 diopter.

dipole antenna A centre fed antenna which is approximately half as long as the wavelength of the radio waves it is primarily intended to transmit or receive.

direct action 1. A controller in which the value of the output signal increases as the value of the input (measured variable or controlled variable) increases. 2. An actuator that extends the actuator stem when the power supply increases.

direct digital control (DDC) A computer control technique that sets the final control element's position directly by the computer output.

direct memory access (DMA) A method of fast data transfer between the peripherals and the computer memory. The transfer does not involve the CPU.

discrete Pertaining to distinct elements or to representation by means of distinct elements, such as characters.

discrete control On/off control. One of the two output values is equal to zero. discrete increment Providing an output which represents the magnitude of the measurand in the form of discrete or quantised values.

discrete input Inputs having a separate and distinct identity. A digital input that is either on or off.

discrete output Outputs having a separate and distinct identity.

displacement 1. The change in position of a body or point with respect to a reference point. 2. The volume swept out by a piston as it moves inside a cylinder from one extreme of its stroke to the other extreme.

displacement meter A meter that measures the amount of a material flowing through a system by recording the number of times a vessel or cavity of known volume is filled and emptied.

displacement type density meter A device that measures liquid density by means of a float and balance beam used in conjunction with a pneumatic sensing system; the float is confined within a small chamber through which the test liquid continually flows, so that density variations with time can be determined.

displacer type liquid level detector A device for determining liquid level by means of force measurements on a cylindrical element partly submerged in the liquid in a vessel.

displacer type meter An apparatus for detecting liquid level or determining gas density by measuring the effect of the fluid on the buoyancy of a displacer unit immersed in it.

dissociation The process by which a chemical compound breaks down into simpler constituents, as the C0₂ and H₂0 at high temperature.

dissolved gases Gases which are "in solution" in water. dissolved solids Those solids in water which are in solution.

distillate 1. The distilled product from a fractionating column. 2. The overhead product from a distillation column. 3. In the oil and gas industry the term distillate refers to a specific product withdrawn from the column, usually near the bottom.
distillate fuel Any of the fuel hydrocarbons obtained during the distillation of petroleum which have boiling points higher than that of gasoline.
distillation vapourisation of a substance with subsequent recovery of the vapour by condensation.

distributed In a control system, refers to control achieved by intelligence that is distributed about the process to be controlled, rather than by a centrally located single unit.

distributed control system (DCS) Instrumentation system consisting of input/output devices, control devices and operator interface devices which in addition to executing the stated control functions also permits transmission of control, measurement, and operating information to and from multiple locations, connected by a communication link.

distributed processing Interconnection of two or more computers so that they can work together on the same problem, not necessarily under the direction of a single control program.

disturbance resolution The minimum change caused by a disturbance in a measured variable which will induce a net change of the ultimately controlled variable.

disturbance variable A measured variable that is uncontrolled and that affects the operations of the process.

Division 1 The (US) classification assigned to a location where either there is a high probability of a dust hazardous atmosphere occurring frequently, or regularly, or where the dust is electrically conductive.

Division 2 The (US) classification assigned to a location where there is a low probability of a dust hazardous atmosphere occurring and/or a high probability of the presence of a hazardous dust layer

Dodge Romig tables A set of standard tables with known statistical characteristics that are used in lot tolerance and AOQL acceptance sampling.

Doppler effect flowmeter A device that uses ultrasonic techniques to determine flow rate; a continuous ultrasonic beam is projected across fluid flowing through the pipe, and the difference between incident beam and transmitted beam frequencies is a measure of fluid flow rate.

Doppler shift A phenomenon that causes electromagnetic or compression waves emanating from an object to have a longer wavelength if the object moves away from an observer than would be the case if the object were stationary with respect to the observer, and to have a shorter wavelength if the object moves toward the observer; it is the physical phenomenon that forms the basis for analysing certain sonar data and certain astronomical observations.

doseThe amount of radiation received at a specific location per unit area or unit volume, or the amount received by the whole body.

dose rate Radiation dose per unit time.

dosimeter (dosemeter) An instrument for directly measuring the total dose of radiation received in a given period.

double-acting Acting in two directions, as in a reciprocating compressor or valve actuator where each piston has a working chamber at both ends of the cylinder.

double precision 1. Pertaining to the use of two computer words to represent a number. 2. In floating point arithmetic, the use of additional bytes or words representing the number, in order to double the number of bits in the mantissa.

dowtherm A constant boiling mixture of phenyl oxide and diphenyl oxide used in high temperature heat transfer systems (boiling point 494°F, 257°C).

dp cell A pressure transducer that responds to the difference in pressure between two pressure sources. Frequently, a diaphragm capsule and an integral part of a dp transmitter. Often used to measure flow by the pressure difference across a restriction in the flow line and level by measuring the pressure difference between the head pressure produced by the height of a liquid in a vessel or tank and a reference pressure.

D/P transducer Transducer that measures differential pressure and converts that to another signal.

draft gauge A type of manometer used to measure small gas heads, such as the draft pressure in a furnace.

draft loss A decrease in the static pressure in a boiler or furnace due to flow resistance.

drag body flow meter A device that measures the net force on a submerged solid body in a direction parallel to the direction of flow, and converts this value to an indication of flow or flow rate.

driving point impedance The complex ratio of applied sinusoidal voltage, force or pressure at the driving point of a transducer to resulting current, velocity or volume velocity, respectively, at the same point, all inputs and outputs being terminated in some specified manner.

driving point reactance The imaginary component of driving point impedance.

driving point resistance The real component of driving point impedance.

droop rate The rate at which the voltage output of a storage device decays.

drop leg The section of measurement piping below the process tap location to the instrument.

dry bulb temperature The temperature of the air indicated by thermometer not affected by the water vapour content of the air.

dry corrosion Atmospheric corrosion taking place at temperatures above the dew point.

dry gas Gas containing no water vapour.

dry steam Steam containing no moisture. Commercially dry steam containing not more than one half of one percent moisture.

dry test meter A type of meter used to determine gas flow rates for billing purposes and to calibrate other flow measuring instruments; it has two chambers separated by a flexible diaphragm which is connected to a dial by means of a gear train; in operation, the chambers are filled alternately, with a flow control valve switching from one chamber to the other as the first becomes completely filled, while flow rate is indicated indirectly from movement of the diaphragm.

dual-sealing valve A valve which uses a resilient seating material for the primary seal and a metal to metal seat for a secondary seal.

duplex control A control in which two independent control elements share a common input signal for the operation of separate final control elements both of which influence the value of the controlled condition.

duplex, full Method of operation of a communication circuit where each end can simultaneously transmit and receive.

duplex, half Permits one direction, electrical communication between stations. Technical arrangements may permit operation in either direction but not simultaneously.

dust 1. Any finely divided solid material 420 nm or smaller in diameter. 2. Particles of gas borne solid matter larger than one micron in diameter.

dust, combustible Dust that (when mixed with air in certain proportions) can be ignited and will propagate a flame.

dynamic characteristics Those characteristics of a transducer which relate to its response to variations of the measurand with time.

dynamic compensation A control technique used to compensate for dynamic response differences to different input streams to a process. A combination of lead and lag algorithms will handle most situations.

dynamic optimisation A type of control, frequently multivariable and adaptive in nature, which optimises some criterion function in bringing the system to the setpoints of the controlled variables. The sum of the weighted, time absolute errors is an example of a typical criterion function to be minimised.

dynamic pressure The increase in pressure above the static pressure that results from complete transformation of the kinetic energy of the fluid into potential energy.

dynamic programming In operations research, a procedure for optimisation of a multistage problem wherein a number of decisions are available at each stage of the process. Contrast with "convex programming," "integer programming," "linear programming, " "mathematical programming, " "nonlinear programming, " and "quadratic programming."

dynamic RAM Random access memory that must be refreshed periodically. Usually faster than "Static RAM "

dynamic range The difference between the highest signal level that will overload the instrument and the lowest signal level that is detectable. Dynamic range is usually expressed in decibels.

dynamic response The behavior of the output of a device as a function of the input, both with respect to time.

dynamic stability The property which permits the response of a positively damped physical system to asymptotically approach a constant value when the level of excitation is constant.

dynamic stiffness The apparent stiffness of a spring member under vibration or shock loading. This apparent stiffness is frequency dependent.